[Gluster-users] Optimizing write performance to a few large files in a small cluster

[Alexander Valys]

A quick performance question.

I have a small cluster of 4 machines, 64 cores in total.  I am running a scientific simulation on them, which writes at between 0.1 and 10 MB/s (total) to roughly 64 HDF5 files.  Each HDF5 file is written by only one process.  The writes are not continuous, but consist of writing roughly 1 MB of data to each file every few seconds.    

Writing to HDF5 involves a lot of reading the file metadata and random seeking within the file,  since we are actually writing to about 30 datasets inside each file.  I am hosting the output on a distributed gluster volume (one brick local to each machine) to provide a unified namespace for the (very rare) case when each process needs to read the other's files.  

I am seeing somewhat lower performance than I expected, i.e. a factor of approximately 4 less throughput than each node writing locally to the bare drives.  I expected the write-behind cache to buffer each write, but it seems that the writes are being quickly flushed across the network regardless of what write-behind cache size I use (32 MB currently), and the simulation stalls while waiting for the I/O operation to finish.  Anyone have any suggestions as to what to look at?  I am using gluster 3.4.2 on ubuntu 12.04.  I have flush-behind turned on, and have mounted the volume with direct-io-mode=disable, and have the cache size set to 256M.  

The nodes are connected via a dedicated gigabit ethernet network, carrying only gluster traffic (no simulation traffic).

(sorry if this message comes through twice, I sent it yesterday but was not subscribed)